If you want to convert a Color image to Grayscale without creating a blotchy image, use this color calculation:
<?php
function imagetograyscale($im)
{
    if (imageistruecolor($im)) {
        imagetruecolortopalette($im, false, 256);
    }

    for ($c = 0; $c < imagecolorstotal($im); $c++) {
        $col = imagecolorsforindex($im, $c);
        $gray = round(0.299 * $col['red'] + 0.587 * $col['green'] + 0.114 * $col['blue']);
        imagecolorset($im, $c, $gray, $gray, $gray);
    }
}
?>

I've been looking for a simple function which "colorizes" an image, without any success ... it looks like a lot of people mean something different with "colorizing" an image, because actually colorzing means multiplying the old color's grayscale with the new color. So a white pixel would become 100% of the colorize-to-color and a black pixel would stay black (I know I can't explain well ... I hope you understood, otherwise take a look at the example below the functions' code).

<?php
function image_colorize(&$img,$rgb) {
  imageTrueColorToPalette($img,true,256);
  $numColors = imageColorsTotal($img);

  for ($x = 0; $x < $numColors; $x++) {
    list($r,$g,$b) = array_values(imageColorsForIndex($img,$x));

    // calculate grayscale in percent
    $grayscale = ($r + $g + $b) / 3 / 0xff;

    imageColorSet($img,$x,
      $grayscale * $rgb[0],
      $grayscale * $rgb[1],
      $grayscale * $rgb[2]
    );
  }

  return true;
}
?>

Example of usage:
<?php
$color = array(0xff,0xaa,0x2a); // color to convert to
$url = 'http://sundog.net/images/uploads/1_google_logo.jpg';
$img = imageCreateFromJpeg($url);

image_colorize($img,$color);

header('Content-type: image/gif');
imageGif($img);
exit;
?>

Enjoy

this is helpful if you would like to implement a color theme system in your website... try it out
Davide Candiloro Italy

function colorize ($pngpath, $r, $g, $b) 
/*
REQUIRES: $pngpath to be a valid path of a greyscale PNG-8 image with 64 colors palette
          $r, $g, $b to be integers in the range 0..255
EFFECTS: returns the png image colorized with the color represented by $r, $g, $b.
*/
{
    header("Content-type: image/png");
    $im = imagecreatefrompng("images/table.png");

    imagetruecolortopalette($im, FALSE, 256); 
   
      for ($c = 0; $c < 64; $c++){ /*64 is the number of colors in the PNG-8 palette*/
        $col = imagecolorsforindex($im, $c);           
        imagecolorset ( $im, $c, $r*$col['red']/256, $g*$col['green']/256, $b*$col['blue']/256); /*replaces original greyscale palette with a colorized one*/
     }

    imagepng($im);
    imagedestroy($im);
}

here's a function to greyscale an image even from a truecolor source (jpeg or png).

slightly poor quality, but very fast...

function imagegreyscale(&$img, $dither=1) {    
    if (!($t = imagecolorstotal($img))) {
        $t = 256;
        imagetruecolortopalette($img, $dither, $t);    
    }
    for ($c = 0; $c < $t; $c++) {    
        $col = imagecolorsforindex($img, $c);
        $min = min($col['red'],$col['green'],$col['blue']);
        $max = max($col['red'],$col['green'],$col['blue']);
        $i = ($max+$min)/2;
        imagecolorset($img, $c, $i, $i, $i);
    }
}

Here is a function to colorize a picture gradient style.

All you have to do is to pass an img-object and an array of colors.

e.g.

$arr = array('#000000', '#990000', '#00FFFF', '#FFFFDD');
colorize ($img, $arr);

<?php
function colorize($imgdata, $palette)
{
    imageTrueColorToPalette($imgdata,false,0xFF);
    $l = count($palette)-1;
    $i = imagecolorstotal($imgdata);
    while ($i--)
    {
        list($r,$g,$b) = array_values(imageColorsForIndex($imgdata,$i));
        
        $grayscale = ($r*.3 + $g*.59 +$b*.11) / 0xFF;
        
        $pos = $l*$grayscale;
        
        $perc = $pos-floor($pos);
        
        $tbase = str_replace("#", '', $palette[$pos]);
        $baseR = hexdec(substr($tbase,0,2));
        $baseG = hexdec(substr($tbase,2,2));
        $baseB = hexdec(substr($tbase,4,2));
        
        $tmix = str_replace("#", '', $palette[$pos+1]);
        $mixR = hexdec(substr($tmix,0,2));
        $mixG = hexdec(substr($tmix,2,2));
        $mixB = hexdec(substr($tmix,4,2));
        
        $resR = $baseR+($mixR-$baseR)*$perc;
        $resG = $baseG+($mixG-$baseG)*$perc;
        $resB = $baseB+($mixB-$baseB)*$perc;
        
        imagecolorset($imgdata, $i, $resR, $resG, $resB);
    }
}
?>

If you are creating a palette image from scratch you have to use imagecolorallocate() for each index before you can use imagecolorset() on it.

I had the same problem as heavyraptor2 so i made this function...
<?php
function imagecolorize($im,$endcolor){
    //funcion takes image and turns black into $endcolor, white to white and
    //everything in between in corresponding gradient
    //$endcolor should be 6 char html color

    //make sure it has usable palette
    if (imageistruecolor($im)) {
        imagetruecolortopalette($im, false, 256);
    }

    //first make it gray to be sure of even results (thanks moxleystratton.com)
    //comment this loop if you want the output based on, for example, 
    //the red channel (for this take a look at the $gray-var in the last loop)
    for ($c = 0; $c < imagecolorstotal($im); $c++) {
        $col = imagecolorsforindex($im, $c);
        $gray = round(0.299 * $col['red'] + 0.587 * $col['green'] + 0.114 * $col['blue']);
        imagecolorset($im, $c, $gray, $gray, $gray);
    }

    //determine end-colors in hexdec
    $EndcolorRGB['r'] = hexdec( substr($endcolor, 0, 2));
    $EndcolorRGB['g'] = hexdec( substr($endcolor, 2, 2));
    $EndcolorRGB['b'] = hexdec( substr($endcolor, 4, 2));

    //determine gradient-delta's
    $stepR = (255-$EndcolorRGB['r'])/255.0;
    $stepG = (255-$EndcolorRGB['g'])/255.0;
    $stepB = (255-$EndcolorRGB['b'])/255.0;

    //aColor contains the 256 gradations between endcolor(i=0) and white(i=255)
    $aColor = array();
    for ($i = 0; $i<=255; $i++){
        $aColor[$i]['r'] = $EndcolorRGB['r'] + ($i*$stepR);
        $aColor[$i]['g'] = $EndcolorRGB['g'] + ($i*$stepG);
        $aColor[$i]['b'] = $EndcolorRGB['b'] + ($i*$stepB);
    }

    //for every color-index we now replace $gray-values for $aColor
    for ($c = 0; $c < imagecolorstotal($im); $c++){
        $currentColorRGB = imagecolorsforindex($im, $c);
        $gray = $currentColorRGB['red'];//image is grayscale, so red,green and blue
        // should be equal. We use this number as key of aColor
        imagecolorset($im,$c,(int)$aColor[$gray]['r'], (int)$aColor[$gray]['g'], (int)$aColor[$gray]['b']);
    }
}?>

This function change colors of an image with ranges 0>100 for each primary color:

int ImageSelectiveColor (int im, int red, int green, int blue)

im is image pointer
red, green and blue are ranges 0->100

function ImageSelectiveColor($im,$red,$green,$blue)
 {
  for($i=0;$i<imagecolorstotal($im);$i++)
   {
    $col=ImageColorsForIndex($im,$i);
    $red_set=$red/100*$col['red'];
    $green_set=$green/100*$col['green'];
    $blue_set=$blue/100*$col['blue'];
    if($red_set>255)$red_set=255;
    if($green_set>255)$green_set=255;
    if($blue_set>255)$blue_set=255;
    imagecolorset($im,$i,$red_set,$green_set,$blue_set);
    
   }
  return $im;
 
 }

Here is a function to turn an image into pure black and white

<?php

function imagepurebw( $img, $amount = 383 ) {
    $total = imagecolorstotal( $img );
    for ( $i = 0; $i < $total; $i++ ) {
        $index = imagecolorsforindex( $img, $i );
        array_pop( $index );
        $color = ( array_sum( $index ) > $amount ) ? 255 : 0;
        imagecolorset( $img, $i, $color, $color, $color );
    }
}

?>

here's a simple function to greyscale an image...

function imagecolorgrey( &$img ) {
 for( $i=0; $i<imagecolorstotal( $img ); $i++ ) {
  $c = ImageColorsForIndex( $img, $i );
  $t = ($c["red"]+$c["green"]+$c["blue"])/3;
  imagecolorset( $img, $i, $t, $t, $t );    
 }
}